EFFECT OF FILM COOLING HOLE AREA RATIO ON ADIABATIC FILM COOLING EFFECTIVENESS OVER A FLAT PLATE

In the present study the film cooling performance is analyzed for a flat plate model with different hole area ratios. Here "Area Ratio" is defined as the ratio between the cross sectional area of the hole at inlet to the exit of the hole. The area ratios which are considered for analyzing are 1, 2 and 3. The models are made in a 10 mm thickness flat plate with a row of 7 holes having 5 mm diameter and at an angle of 22 degrees towards the downstream. The first model has an area ratio of 1 with a constant cross sectional area throughout the hole. For the second and third models, the area of hole exit is diverged to the corresponding area ratios of 2 and 3 from the mid of the plate. The divergence of hole is made with the area extended towards the downstream. A stainless steel sheet with a low thermal conductivity substrate is placed at the downstream of test model. To study the film cooling performance, two parameters have been considered namely, convective heat transfer coefficient and adiabatic film cooling effectiveness. Both the film effectiveness and convective heat transfer coefficients can be obtained from the same test plate setup. The heat transfer coefficient is obtained by constant heat flux boundary condition over the stainless steel sheet by heating it through current supply. For the film cooling effectiveness measurement, the mainstream is kept at atmospheric temperature and the coolant is maintained at a temperature to attain a constant density ratio equal to 1.3. Experiments are carried out at various blowing ratios in the range of 0.5 to 2. The temperature measurement over the flat plate is measured through an IR camera. Among the considered three models, the model with the higher area ratio had shown higher adiabatic film cooling effectiveness. On analyzing the heat transfer coefficient distribution, no significant changes were found for all the models.